#include <bits/stdc++.h>
#define ll long long
#define str string
#define ins insert
#define ld long double
#define pb push_back
#define pf push_front
#define pof pop_front()
#define pob pop_back()
#define lb lower_bound
#define ub upper_bound
#define endl "\n"
#define fr first
#define sc second
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define sz size()
#define vll vector<ll>
#define bc back()
#define arr array
#define pll vector<pair<ll,ll>>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define ordered_set tree<ll, null_type,less_equal<ll>, rb_tree_tag,tree_order_statistics_node_update>
template<class S,class T>
bool chmin(S &a,const T &b) {
return a>b?(a=b)==b:false;
}
template<class S,class T>
bool chmax(S &a,const T &b) {
return a<b?(a=b)==b:false;
}
//void fre(string s){freopen((s+".in").c_str(),"r",stdin);freopen((s+".out").c_str(),"w",stdout);}
void start(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
}
const ll inf=1e18;
const ll mod=998244353;
const ll N=1e5+7;
const ll M=1e5+7;
const ld eps=1e-9;
ll val[N];
ll dis(ll i,ll j){
return 1;
if(i==0)return 1;
return val[j]-val[i];
}
void solve(){
ll i,j;
ll n,k;
cin>>n>>k;
pll v(n);
set<ll> st;
map<ll,ll> ccnt,mp;
for(i=0;i<n;i++){
cin>>v[i].fr>>v[i].sc;
ccnt[v[i].fr]++;
}
sort(all(v));
ll x=0,c=0;
while(true){
c+=ccnt[x];
if(c==0){
ll low=ub(all(v),make_pair(x,-inf))-v.begin();
if(low==v.sz) break;
x=v[low].fr;
continue;
}
st.ins(x);
c--;
x++;
}
c=0;
for(auto i : st){
mp[i]=++c;
val[c]=i;
}
vll cnt(n+5,0);
ll dp[n+5][n+5][n+5];
for(i=0;i<=n+1;i++){
for(j=0;j<=n+1;j++){
for(ll q=0;q<=n+1;q++)dp[i][j][q]=inf;
}
}
vll mn(n+5,inf);
st.clear();
for(i=0;i<n;i++){
v[i].fr=mp[v[i].fr];
cnt[v[i].fr]++;
st.ins(v[i].sc);
mn[v[i].fr]=*st.begin();
}
for(i=2;i<=n;i++){
mn[i]=min(mn[i],mn[i-1]);
}
dp[0][0][1]=0;
for(i=0;i<=c;i++){
for(j=0;j<=n;j++){
for(ll q=1;q<=n;q++){
if(dp[i][j][q]>=inf) continue;
for(ll l=0;l<=j;l++){
if(mn[i-1]==inf && l>0)break;
ll x=j-l;
chmin(dp[i+1][max(0ll,cnt[i+1]+x-(q+l))][q+l],dp[i][j][q]+x*k*dis(i,i+1)+(i>0 ? l*mn[i-1] : 0));
}
}
}
}
ll ans=inf;
for(i=0;i<=n;i++){
chmin(ans,dp[n+1][0][i]);
}
cout<<ans<<endl;
}
signed main(){
start();
ll t=1;
//cin>>t;
while(t--) solve();
return 0;
}
/*
7 10
1 2
1 3
4 2
100 1
100 2
100 3
300 2
*/
